Means for electrifying pulverulent materials



Nov. 17, 1953 P. HAMPE 2,659,841

MEANS FOR ELECTRIFYING PULV-ERULENT MATERIALS Filed July 22, 1949 3ploying high voltages which may, for example, carry voltages of theorder of 1,500 to 2,000 volts per millimeter of distance betweenelectrodes.

It is a feature of the invention that full advantage may be taken of lowvoltage operation. In my apparatus, the sparking hazard may be lessenedby the provision of a high resistance in the feed circuit positionedjust ahead of the discharge electrode. Such a resistance not only servesto prevent sparking but prevents glowing discharges that may eitherdamage the electrode or ignite the powder particles. Furthermore, myapparatus is advantageously provided with several nozzles, each nozzlebeing provided with a resistance, this arrangement thus serving todivide the discharge of the generator between the various nozzles and toreduce the risk of injury should an operator touch one of the nozzles.

I also advantageously reduce sparking by imparting a high speed to thecurrent of particlecarrying gas conducted through the electrifying zonebetween the electrodes. Alternately I provide means for operating myapparatus at a superatmospheric gas pressure. For a given voltage, thesparking length is inversely proportional to the pressure. A small sizeionizer in accordance with the invention is particularly adapted fortaking advantage of the aforementioned conditions which are conducive toefiicient electrification. Furthermore, in an ionizer in which there isa relatively small clearance between the electrodes, the time requiredfor the charging of a powder particle is proportionately small. In fact,the time required for obtaining a given proportion of the maximum chargeis inversely proportional to the concentration P of the charges in theionized gas. Thus, the formula shows that the concentration isproportional to the field E and in inverse proportion to the distance Rto the electrode.

In apparatus according to the invention, E may reach a value three timesas great as that possible in heretofore known ionizers and R may be tentimes less, e. g. two millimeters instead of twenty millimeters. Thus, acharging period sixty times smaller is possible and the length of theionizer may, accordingly, be reduced. In View of this fact, it has beenpossible in accordance with the invention to replace the standard finewire electrode by a tip, an arrangement which has not been satisfactoryin known ionizers of larger size. On the other hand. the high value ofthe charge on the particles enables the ionizer to be fed with gas thatis more heavily charged with powder particles without damage of cloggingthe apparatus. '1 have found that in addition to the above mentionedmeasures taken for avoiding sparking, certain other precautionarymeasures should also be taken. In a small ionizer, such as an apparatusmade in accordance with the invention, the two electrodes are moresymmetrical than in larger ionizers. Consequently, the strong field thatprevails between the electrodes has a tendency to cause eddy currents toappear on the electrode of larger surface. These electrical dischargesof opposite polarity discharge the powder and they are created if theelectrode becomes covered with powder, particularly if the powder is ofaninsulating nature. In practice, the strong field which is set up bythe ionizer has a tendency to create a strongly attractive deposit thateven a rapid currentof gas is not effective to prevent.

4 Accordingly, one of the features of the apparatus of the invention isthe prevention of the formation of such deposits on the electrodes.There are several means for advantageously preventing these deposits.

One means comprises the addition to the powder to be electrified of anabrasive such as alumina or quartz. In addition, the nozzle of theionizer may be shaped to facilitate the cleaning process by insuringthat no portion of the surface is sheltered from impacts of the flOWinggas. The above mentioned means are particularly effective for insuringthat the smaller electrode will be kept clean. Other means areparticularly suitable for cleaning the large surface electrode. Thus, inone embodiment of the invention, the larger surface electrode may bearranged to slide in such a way that its inner surface is cleaned by afixed surface or the larger electrode may take the form of a revolvingmember which is cleaned by a fixed brush, constantly in engagement withits surface. The cleaning means is located out of the space between theclosest portions of the electrodes so that they do not cause arc-overs.

In order to increase the electrical field in the apparatus and toprovide the particles with a higher charge, it is advantageous to causethe charge to take place in the.presence of compressed gas. Inaccordance with one embodiment of the invention, therefore, means areprovided for feeding the apparatus with gas at a pressure up to severalatmospheres, the larger surface electrode being advantageously formed insuch manner that the pressure is maintained during the movement of theparticle carrying gas through the electrical field. For creating highgas pressures, I advantageously provide compressors, diaphragm pumps andthe like in place of the fan blades which have been used in priorionizer apparatus.

The objects, characteristics and advantages of the invention will bemore fully understood from the following description and claims inconjunction with the accompanying drawings, in which:

Fig. 1 is a longitudinal section of an ionizer in accordance with theinvention in which an end portion of the nozzle is slidable relative toa fixed portion to keep the nozzle clean during operation of theapparatus.

Fig. 2 is a side elevation, partially in section, of an ionizer in whichthe larger surface electrode is formed by rotating members.

Fig. 3 is a front view of the arrangement shown in Fig. 2.

The apparatus illustrated in Fig. 1 comprises a central electrode l 0which is shown as a pointed wire electrode having a sharp point H. Thepointed electrode I0 is connected through a high resistance R to adirect current voltage supply HT the voltage of which is preferably inthe range of 3,000 to 6,000 volts. A second electrode l2 surrounds thetip portion I I of the central electrode l0 and comprises a section oftubing that is coaxial with the central electrode. The tubing section I2is telescopically slidable on the end portion of a fixed tube l3 throughwhich a stream of air or other gas carrying the particles of pulverulentmaterial to be charged is supplied.

The tubing l2, I3 is connected to ground, as indicated at G. Thus, anelectric field is produced between the tip portion of the centralelectrode l0 and the inner surface of the tubing section l2 forming theouter electrode. This inner surface may thus be referred to as theactive surface of the electrode I2. The stream of gas carrying thepulverulent material to be har ed ws hrou h he t in 2, '3 n he di ecti neeted by e a row and hus :fiows t r h he field be we n the electrod sand 1:2, b i th eupon dischar d int th at phere. The discharge end ofthe tubing 12 is preferably flared outwardly, as indicated at 2.

In order-to clean the active inner surface of the tubular electrode 12during operation of the apparatus and thereby keep it clear of particlesof the material being charged, provision is made for reciprocating themovable tubing section 12 relative :to the :fixed .tubing 1:3. "Forexample, as illustrated in the drawings, the tubing section 12 isreciprocated .by a :flexiblecable or Bowden 'wire 14. As the tubingsection 12 is reciprocated, its inner surface rubs on the outer end 123.of the fixed tube 1.3 so as-to dislodge any material that may havecollected on the inner surface of the tubingsect-ion 12. "It will beseen that the end edge :23 of the tubing .13 which constitutescleaning-means forthe inner surface of the tubular electrode 12 isdisposedoutside of the direct field between the closest portions of theelectrode .12 and the .central electrode 10. Hence, the cleaning meansdoes not reduce the gap :between the electrodes or tend to cause anyarc-over between the electrodes.

in Figs. '2 and 3, there is shown a preferred form of apparatus 'inaccordance with the invention in which the outerelectrode comprises twogrooved pulleys .15 and i6 rotatable about spaced parallel axes so thatthe peripheries of the pulleysare close-to one another. The grooves inthe peripheries of the pulleys 15, 16 are approximately semi-circular incross section so as to provide a space 20 of approximately circularcross section between thetwo-pulleys. A pointed centralelectrode 19projects into, and is concentric with, this circular-space so as to beat least partially surrounded bythe outer electrode composed of thegrooved pulleys 15, 16. As in the foregoing embodiment, the centralelectrode 18 is connected through a resistance R with a-direct currenthigh voltage source to provide an electric-fieldbetweenthe pulleys 1-5,1-6and the pointed central electrode 19. A tube 21 blows a powder-loadedair stream through thespace 20 between the pulleys 1-5, 16 and thusthrough the above mentioned electrical field.

During operationof the apparatus, "the grooved pulleys and 116 arerotated in the directions indicated by the arrows by any suitabledriving means. Contact brushes 1.! and 18engage the grooved peripheriesof the pulleys and keep the grooves clean as the pulleys are rotated.The pulleys 155, 16, contact brushes 1'1, 18 and supply tubeare-grounded as indicated at G.

.As aimodifie iema ng ul maybe e instead .of two, thispulley beingcleaned in similar-manner by a grounded contact brush.

-It will beseen that, in the embodiment of-Figs. and 3, a .n that .of Fi.l. the u l trod is movable relative tofixed clean n means to keep theactive. surface of said-electrode clean during operationof theapparatus. By thus keeping this surface clean, much closer spacing ofthe electrodes can be used than would otherwise be possible. Since thecleaning means is located outside the direct field between the closestportions of the inner and outer electrodes, it does not in any wayinterfere with the desirable small electrode gap.

In addition to keeping the active surface of the outer electrode clean,as described above, it is also important to keep the central electrodeclean.

B st sul s have been btain d by her 1 the entral l ctrode p int in a dre t n ,QPPQ e to thatof the flow of t-hegaseous cur-rent. gut U way,the central electrode is kept clean .andthere has also been found to bea more desirable distribution of the ions in gradually charging thepowder during the short length of time it takes to pass through theionizer. Cleaning .of the central electrode is also improved by drawingout the tip to a very fine point and by using a hard noneoxidizablemetal such vas tungsten, nickel, chromium, etc.

It is also absolutely necessary to protect the point against sparkingbetween the point and the surface of the .outerelectrode. Such sparkingis inhibited by the use of a resistance between the centralelectrode andthe voltage supply.

In order to assure the satisfactory working of the apparatus, it isdesirable for the air cur;- rent to travel at azhigh speed. This isachieved by supplying the ionizer with air compressed to a suitablepressure. For example, it has been found that good results are obtainedwith pres-.- sures of 20'to 40 grams per square centimeter. Previousdevices-employing a fan or zblower have operated with only 1 to -5 gramsper square centimeter pressure. -With a view :to increasing the voltageof the electrical field :in the ionizer and thereby transmit to thepowders a "higher charge, devices in accordance with the invention mayoperate with an air compressed up-to several atmospheres, the dischargeopening ornozzlebeing restricted so as to provide superatmosphericpressure at the point where electrification takes place.

Apparatus in accordance with the present invention preferably usesdirect current with a voltage of the order of 3,000 to 6,000 volts andan output per unit of about microamperes. The required voltage may besupplied by a small generator. Especially suitable are induction coil ortransformer generators provided with electronic, copper oxide orrotating commutator-rectifiers, the primary current being supplied by a.small alternator orby a battery. In this way, dusting equipment isobtained that is easy to carry and is especiall suitable foragricultural purposes. The weight of the electric generator requiredisof the order of one kilogram andthe electricpower requirement is lessthan one watt.

"It willibe understood that the apparatus shown in the drawings ismerely by way of example use that the invention is not limited to thespecific apparatus herein particularly shown and described.

In the claims:

n apparatus o p tin highly el c r fied particles of pulverulentmaterial, an outer electrode comprising a pair of grooved pulleysrotatable about spaced axes and having their peripheries .close to oneanother, peripheral grooves in said pulleys forming ,aspaceqfapproximately circular cross-section .between said pulleys, aninner electrode comprising a pointed wire disposed between saidpulleysand .means for directing a jet or gas carrying said particlesaround said inner electrode and between said grooved pulleys.

2. In apparatus for projecting highly electrified particles ofpulverulent material, an outer electrode Comprising a pair of groovedpulleys rotatable about spaced axes and having their peripheries closeto one another, peripheral grooves in said pulleys forming a space ofapproximately circular cross-section between said pulleys, an innerelectrode comprising a pointed wire disposed between said pulleys, meansfor directing a jet of gas carrying said particles around said innerelectrode and between said grooved pulleys and means for cleaning thegrooved peripheries of said pulleys as they rotate.

3. Apparatus for projecting highly electrified particles of pulverulentmaterial into the atmosphere, comprising a pointed wire electrode and asecond electrode having an active surface of materially large areafacing and at least partially surrounding the pointed electrode, thedistance between the electrodes being of the order of a few millimeters,means for producing an electric field between the pointed electrode andsaid active surface of the large-surface electrode, the potentialbetween said electrodes being of the order of 2500 to 8000 volts, meansfor directing a stream of gas carrying said particles through theelectric field between said electrodes and into the atmosphere, andfixed rubbing means engaging said active surface of said large-surfaceelectrode, said large-surface electrode being movable relative to saidfixed rubbing means during the electrification and projection of saidparticles to bring substantially the entire area of said active surfaceinto engagement with said rubbing means and thereby keep said activesurface clear of said particles, said rubbing means being disposed outof the direct electric field between the closest portions of saidelectrodes.

4. Apparatus according to claim 3, in which said pointed wire electrodepoints in a direction opposite to the direction of flow of said streamof gas.

5. Apparatus for projecting highly electrified particles of pulverulentmaterial into the atmosphere, comprising a pointed wire electrode and asecond electrode having an active surface of substantially larger areafacing and at least partially surrounding said pointed electrode, saidactive surface being a surface of revolution and the distance betweensaid electrodes being of the order of a few millimeters, means forproducing an electric field between the pointed electrode and saidactive surface of the large surface electrode, the potential betweensaid electrodes being of the order of 2500 to 8000 volts, means fordirecting a stream of gas carrying said particles through the electricfield between said electrodes and into the atmosphere, and means forcleaning particles of material from said active surface, saidlarge-surface electrode being rotatable about the axis of said surfaceof revolution during the electrification and projection of saidparticles to bring substantially the entire area of said active surfaceprogressively into engagement with said cleaning means to keep saidsurface clear of said particles, said cleaning means being disposed outof the space between the closest portions of said electrodes.

6. Apparatus for projecting highly electrified particles of pulverulentmaterial into the atmosphere, comprising a pointed wire electrode and asecond electrode comprising a pair of rotatable members having activesurfaces of substantially larger area facing and at least partiallysurrounding said pointed electrode, said active surfaces being surfacesof revolution and the distance between said surfaces and the pointedelectrode being of the order of a few millimeters, means for producingan electric field between the pointed electrode and said active surfacesof said second electrode, the potential between said electrodes being ofthe order of 2500 to 8000 volts, means for directing a stream of gascarrying said particles through the electric field between saidelectrodes and into the atmosphere, and means for cleaning particles ofmaterial from said active surfaces, said rotatable members being rotatedduring the electrification and projection of said particles to bringsubstantially the entire area of said active surfaces progressively intoengagement with said cleaning means to keep said surfaces clear of saidparticles, said cleaning means being disposed out of the space betweenthe closest portions of said electrodes.

7. Apparatus for projecting highly electrified particles of pulverulentmaterial into the atmosphere, comprising a pointed wire electrode and asecond electrode having an active surface of substantially larger areafacing and at least partially surrounding the pointed electrode, thedistance between the electrodes being of the order of a few millimeters,means for producing an electric field between the pointed electrode andsaid active surface of the large-surface electrode, the potentialbetween said electrodes being of the order of 2500 to 8000 volts, meansfor directing a stream of gas carrying said particles through theelectric field between said electrodes and into the atmosphere, meansfor cleaning particles of material from said active surface, said secondelectrode being reciprocable relative to the pointed electrode and saidcleaning means, and means for reciprocating said second electrode duringthe electrification and projection of said particles to bringsubstantially the entire area of said active surface into engagementwith said cleaning means to keep said surface clear of said particles,said cleaning means being spaced from said pointed electrode in thedirection of reciprocation of said second electrode so as to be disposedout of the space between the closest portions of said electrodes.

PIERRE HAMPE.

References Cited in the file of this patent UNITED STATES PATENTS NumberName Date 1,444.092 Anderson Feb. 6, 1923 1,928,963 Chaffee Oct. 3, 19332,302,289 Bramston-Cook Nov. 17, 1942 2,313,389 Meston Mar. 9, 19432,396,533 Root Mar. 12, 1946 2,477,947 Yadoff Aug. 2, 1949 2,484,202Wintermute Oct. 11, 1949 2,509,277 Ransburg May 30, 1950 2,526,178 WeberOct. 17, 1950

